1. Field of Invention
The present invention relates to a method for producing a polyimide film, more particularly, to a method for producing a polyimide film by means of controlling its adhesive property.
2. Disclosure of the Related Art
As is commonly known, a polyimide film has various excellent properties such as heat-resistance, cold resistance, chemical-resistance, insulation and mechanical strength. Therefore, a polyimide film is widely used as such materials as an electric insulating film, a heat insulating film and a base film of flexible printed wiring board. In order to use a polyimide film for various purposes, a copper-clad laminated board is produced by superposing a layer of adhesive agent and copper foil on one or a both sides of a polyimide film, a pre-impregnated film (prepreg) is produced by coating a polyimide film with adhesive agent, or a composite film is produced by laminating fluorocarbon resin on a polyimide film. The adhesive strength is one of the important properties of a film, so that various attempts to improve the adhesive strength of a film have been made.
In order to provide the adhesive strength to a surface of a polymeric film, there are various techniques such as flame treatment for a film surface, corona discharge treatment, ultraviolet treatment, alkaline treatment, primer treatment, sand blast treatment and plasma treatment. For the purpose of providing the adhesive strength to a polyimide film without spoiling its other properties, one or more of the above-mentioned treatments are selected. These treatments are considered to be used for removing a weak boundary layer, which is formed on the film surface in the course of solvent-cast process and lowers the adhesive strength of a film.
All of the above-mentioned treatments are executed after producing a film to improve the adhesive strength of a finished film. The adhesive strength of a finished film is not constant. The above-mentioned after-treatments are also unstable. Therefore, it is difficult to stably provide a film with an improved adhesive strength by a prior art method. There is also an inevitable problem that the cost goes up due to the additional step to improve an adhesive property of a finished film. Same troubles and problems also occur in the course of producing a composite of a polyimide film and fluorocarbon resin. Thus, it has been difficult to constantly provide high adhesive strength to a film by using the conventional methods.